This invention relates to method of and apparatus for producing an electrical output signal expressing the ratio between two quantities and involves the generation of a composite electrical signal comprising elemental portions of a signal component representing one quantity alternating with elemental portions of another signal component representing the other quantity so that two series of elemental portions are set up both of which are subject to variable phase shift, provision being made for enabling the said ratio to be computed with substantial cancellation of the adverse effect of said variable phase shift on ratio accuracy.
The invention relates in particular to method of and apparatus for double-beam, ratio-recording, infrared spectrophotometry in which the composite electrical signal is generated by means including a detector which is either of the thermal type, e.g. thermo-couple, or has a signal rise characteristic similar to that of the thermal type.
The generality of the present invention is more readily appreciated in the specific context of the spectrophotometric application referred to in the above statement.
In double-beam, ratio-recording spectrophotometry, the thermo-couple is still the preferred choice for the photometric detector in that its performance, particularly at the long wavelengths of the infrared spectrum, is generally superior to that of any other device that could be considered in practice, including the newly developed pyroelectric detectors. Unfortunately, any thermal detector requires a finite time to heat up and cool down, which inevitably means that the electrical signal it produces lags behind the change in the impinging radiation that has caused it. For the purposes of the present discussion, it will be assumed that detection of the photometric radiation is performed by a thermo-couple.
In the spectrophotometric process under review, the detector is exposed, for equal intervals of time, to the radiation emerging alternately from the sample optical channel and the reference optical channel, with the result that the detector signal includes an elemental portion of a signal component representing sample transmission or absorption (which identifies with one of said quantities) alternating with an elemental portion of a signal component representing reference transmission or absorption (which identifies with the other of said quantities), the two elemental portions being of equal duration. The detector signal is therefore a composite electrical signal in that it includes two distinct series of elemental portions, but because of the sluggish response of the thermo-couple, an elemental signal portion belonging to one series has not completely died away when the next following elemental signal portion of the other series begins to rise.
This leads to what is known in the art as "cross-talk" between the sample signal component and the reference signal component, which has to be taken into account when the detector signal is demodulated in order to separate the two components. In practice, cross-talk is minimized by suitably phasing the demodulation points to the beam switching means through which the detector is alternately exposed to one and other of the two optical channels. In a commercial instrument, the phasing up operation ensuring effective cross-talk cancellation is carried out at the factory but the cancellation can only be truly valid if the phase shift (or phase difference) between the optical pulse impinging upon the detector and the resulting elemental signal portion of the respective series is constant regardless, for example, of the control settings chosen by the operator of the instrument. In the present context, the phrase "phase shift variation" is intended to allude to the fact that a phase shift as defined is present and is subject to change. An important cause of said variations will be presently described.
A more detailed account of how cross-talk arises in the same general spectrophotometric context of the present discussion and of the conditions to be met in order to minimize it is found in U.S. Pat. No. 4,132,481, which is imported in full into the present application. There the disturbance to cross-talk cancellation brought about by mains frequency variations was identified for the first time and a solution given. Without in any way detracting from the generality of the present invention, it may be stated that in so far as its specific application to spectrophotometry is concerned the problem that had to be solved was how to overcome the disturbance to cross-talk cancellation brought about by variable phase shift in the generation of the elemental signal portions of the two series referred to.
There are a number of ways in which the signal generating means including the detector and the electronic system for handling its output may inevitably give rise to phase shift variations. It has been observed earlier on that in infrared spectrophotometry the detector more frequently used in the present state of the art is the thermo-couple. It typically comprises active semi-conductor material in a pair of confined areas bridged by an overlying metal foil of rectangular shape, called the "target", which covers both in length and width a significantly larger area than the total semiconductor active area. The areas are spaced apart along the longitudinal axis of the target and, in the normal operation of the spectrophotometer, they "see" a substantially constant length of the radiation strip impinging upon the target and representing the image of the monochromator exit slit, regardless of the slit opening actually chosen by the user. They do not "see" a constant width, however. In fact, at moderate openings, they are not substantially overfilled by the radiation width and at large openings they are considerably overfilled. In the first case, the radiation will only have to traverse the thickness of the foil in reaching the active areas; in the second case, it must first travel along the plane of the foil as well as traverse its thickness. This means that as the slit opening is increased the elemental signal portions referred to earlier will receive signal contributions that have had less and less time to rise because of the greater thermal impedance met (and consequently greater time delay suffered) by the radiation in reaching the active areas. As a result, the elemental signal portions of both series will rise to a lesser height in the allotted time interval than would have been the case if no significant additional thermal lag had been caused by opening the slits. If the transmission of the analytical sample relative to that of the reference is observed at a given wavenumber and a given slit opening and then at the same wavenumber but an increased opening, a signal change will take place due to a change in phase shift from that associated with the given opening to that associated with the increased opening. The said signal change is, therefore, a cause of photometric inaccuracy because if the sample is not being scanned (and we assume that it is not undergoing physical or chemical changes) there should be no change in the observed sample transmission.
Undesired phase shift variations are also introduced or compounded when the length of the radiation strip impinging on the thermo-couple is restricted, as a result for example of having to use a very small sample in a regular spectrophotometer the optics of which include no provision for restoring the radiation strip to its normal length. Inadequate emission uniformity of the radiation source is another possible cause, and so is poor phase response of the signal processing chain, the latter being a likely cause not only in spectrophotometry but also in other widely differing applications of the present invention.
It can now be appreciated that any phase shift variations, whatever their cause, must have a disturbing effect on the setting for cross-talk cancellation carried out by the manufacturer. Considering for example the slit-opening effect referred to earlier, if the setting is optimum at one moderate slit opening, it is less than optimum when the slits are opened wide, since the rise of the elemental signals will have suffered a delay and the demodulation points as set at the factory may need to be delayed further with respect to the switching of the radiation beam. This particular effect becomes increasingly more serious as the width of the target is increased, or, in other words, as wider and wider maximum openings are allowed for in instrument design. In practice, manufacturers have tended to tolerate it, confining themselves to spreading the resulting photometric error by using a slit opening intermediate between minimum and maximum in the setting up of the spectrophotometer for optimum cross-talk cancellation.
A prior art proposal (U.S. Pat. No. 3,659,942) singles out the slit-opening effect and suggests the use of a servo system which adjusts the demodulation points in response to a function of slit opening. Unfortunately, the system is in no way effective against other causes of disturbance of the cross-talk cancellation setting and in certain instances may even compound them. It does require exact foreknowledge of the detector phase response, which means that a change of detector calls for the generation of a different function of slit opening. The ideal solution would take care of any phase shift variation without even requiring any knowledge of its cause and extent. Viewed against the background of the prior art, such solution would seem most unlikely.